There are plenty of places to buy a dozen pens. Need 12,000, though, and the options dwindle.

That distinction between the retail market for everyday office and school supplies and the big business of supplying corporate and government clients under contract could loom large as regulators weigh
Staples Inc.
’s $6.3 billion deal to buy rival
Office Depot Inc.
ODP 1.20%

The merger, unveiled Wednesday, would take the U.S. down to one chain of office-supply superstores from three in just a couple of years. The Federal Trade Commission waved through Office Depot’s acquisition of OfficeMax in 2013. The current deal may get a closer look.

It is conventional wisdom that the office-supply market for consumers is highly competitive, with prices kept in check by ferocious competition among retailers such as
Wal-Mart Stores Inc.
and
Amazon.com Inc.,
both online and in stores. The FTC validated that view when approving the Office Depot’s deal with OfficeMax.

The market for business customers has received less scrutiny. It wasn’t an issue when the FTC rejected Staples’ first attempt to buy Office Depot in 1997. It wasn’t a sticking point when Office Depot bought OfficeMax, either. But in that deal, part of the FTC’s reasoning is that “the parties will continue to face strong competition for such customers from Staples,” raising questions about the current review.

“This investigation isn’t going to be about where you and I can buy a stapler,” said
Amanda Wait,
a former FTC litigator who now practices at law firm Hunton & Williams LLP. “It’s about where a company can buy 10,000 staplers.”

Staples Chief Executive
Ron Sargent
said on a conference call Wednesday that the company faces a number of competitors in its contract business.

“There are really strong regional players, who bid on contracts all the time, and have been very, very successful,” Mr. Sargent said. “Amazon just launched a business-to-business office products initiative. They’re knocking on the door.”

Leo Meehan,
chief executive of closely held W.B. Mason Co., a regional office-supplies seller, said it was too soon to tell whether the merger would hurt his business. W.B. Mason does about $1.5 billion in business a year, about a third of which comes from large buyers, including government clients, Mr. Meehan said

“Now we’re just going to have this one monstrous competitor,” he said. But “every time there’s been a merger, each one has over time brought opportunity for us. One way or another, we’ll be out there trying.”

Investors appeared to be betting the deal will get real scrutiny from regulators. Office Depot shares closed up 2.2% at $9.49, well below the deal’s value of $10.91 a share at Staples’ closing price Wednesday.

Staples’ shares fell 12% to $16.73

Office Depot and Staples compete fiercely to sell paper, pens, coffee, light bulbs and janitorial supplies to medium-sized and large businesses—clients that buy supplies direct from the chains without ever setting foot in a store.

Those operations account for about 37% of revenue at each company, bringing in $6.2 billion in sales for Staples in the nine months that ended Nov. 1 and $4.6 billion in sales over the nine months through Sept. 27 at Office Depot. They also are holding up better than their retail businesses. Neither companies’ North American same-store sales have grown in years.

Both are already among the 10 biggest online retailers, with Staples coming in No. 3 behind Amazon.com and Apple Inc., and Office Depot at No. 9, according to Internet Retailer, a trade publication.

So far, customers aren’t expressing much concern. Dannon, the U.S. based subsidiary of the global food giant Danone SA, buys its office supplies from Staples.com and isn’t worried it will lose options with the merger. The yogurt maker cites one main reason: e-commerce.

“We already have a lot of other options when we buy at this scale,” spokesman
Michael Neuwirth
said. Dannon likes Staples.com because of the variety and breadth of products they sell. “Not that we have a big variety of Post-it Note colors here,” Mr. Neuwirth said.

Toy company Jakks Pacific Inc. also has a corporate account with Staples, from which it buys janitorial and office supplies, as well as furniture. A spokeswoman said the company wouldn’t be concerned about a combination of the two largest office-supply chains, because there are plenty of places to buy pens and paper.

Market Talk

Office Deal Great for Consumers, Less So for Suppliers On the conference call regarding Staples’s $6.3 billion deal to buy Office Depot, executives at the companies said a combination would allow the new company to compete head-to-head on price with Amazon, Wal-Mart and other low-cost purveyors. While a deal may be good news for consumers, who will likely reap savings, the suppliers to both companies likely view the deal as less of a victory. When retailers combine, they often use their new size and clout to renegotiate contracts with their suppliers to push prices down. The focus on lower prices on the conference call suggests that might be in store for suppliers once a deal closes. (Dana.Mattioli@wsj.com; @DanaMattioli)

Return of the Sole Adviser? In a deal environment where it is the norm to see 3 or 4 banks advising companies on buying other public companies, Staples’ deal to buy Office Depot is a rarity. Each were represented by just one bank on the deal--Barclays for SPLS and boutique investment bank Peter J. Solomon for ODP. BCS is also leading the deal’s financing. The agreement will boost each bank’s standing in league tables as sole-adviser roles often translate into higher fees since the pot doesn’t have to be split with other banks. (Dana.Mattioli@wsj.com; @DanaMattioli)

Market Talk is a stream of real-time news and market analysis that’s available on Dow Jones Newswires

Tougher competition is part of the reason the FTC in 2013 approved Office Depot’s takeover of the smaller OfficeMax. Regulators found each company had little impact on the other’s in-store prices, and they found “dozens, if not hundreds, of office suppliers that compete effectively to serve small and medium-size businesses.” The FTC also found a growing list of regional competitors like W.B. Mason capable of winning big corporate contracts.

On the other hand, the commission also found Office Depot and OfficeMax were “rarely each other’s closest competitor” for large customers and that Staples was there as a counterweight.

If the FTC believes regional competitors like W.B. Mason and Amazon can provide enough competition to keep prices down for corporate and government clients, they may be more likely to approve the merger, experts said. Another factor would be the extent to which companies can buy supplies straight from manufacturers.

Staples has agreed to divest businesses that generated up to $1.25 billion of Office Depot’s U.S. 2014 revenue, but hasn’t committed to a “hell or high water” provision that would require it to do anything necessary to secure antitrust approval—a condition to which buyers are generally reluctant to agree.